This invention relates generally to radiofrequency plasma processing systems and, more particularly, relates to plasma sources for radiofrequency plasma processing systems which improve the efficiency of inductive coupling of radiofrequency energy to the plasma.
Plasmas are widely used in integrated circuit fabrication to modify the surfaces of semiconductor substrates, such as silicon wafers, and in other applications to modify the surfaces of workpieces, such as flat panel displays. Familiar plasma processes include sputter etching, reactive ion etching, ionized physical vapor deposition, plasma-enhanced chemical vapor deposition, surface conditioning, and surface cleaning. Plasma processing generally removes or adds a layer of a material to the substrate, or a patterned area on the surface of the substrate, by either etching, sputtering, growing, or depositing, or chemically modifies a thin surface layer.
Plasmas are generated by coupling excitation energy from an energy source with a sub-atmospheric or vacuum pressure of a process gas confined inside a vacuum chamber. In a widely-practiced method of generating a plasma, radiofrequency (RF) electrical energy is coupled with the process gas to create a rapidly oscillating (i.e., time-varying) electromagnetic field. The oscillating electromagnetic field precipitates a circulating flow of electrons that, in a cascade of individual electron-gas molecule collisions, ionizes the process gas. The plasma of positively and negatively charged particles is sustained by coupling between the RF electromagnetic field and the electrical load presented by the plasma.
Highly-dense plasmas may be generated by inductively coupling the magnetic field component of the RF electromagnetic field to the plasma into the vacuum chamber. Structure, such as a slotted electrostatic shield or a deposition shield, is typically incorporated into an inductively-coupled plasma (ICP) processing system which suppresses capacitive-coupling of the RF energy to the plasma. The deposition shield preferentially transmits the inductively-coupled magnetic component of the RF energy. Inductively-coupled plasmas generally have a low plasma potential. In an ICP processing system, the plasma potential is determined by the characteristics of the electrons in the bulk plasma, which is surrounded by the grounded deposition shield and chamber wall of the vacuum chamber. The ICP processing system lacks active electrodes either inside the vacuum chamber or interfacing with the plasma that could cause time-dependent fluctuations in the level of the plasma potential. Thus, the plasma potential is an intrinsic property of the inductively-coupled plasma and a greater level of RF energy may be inductively coupled with the plasma for enhancing the ion density while retaining a relatively low plasma potential.
In the operation of an ICP processing system, a negative bias potential is usually applied to the substrate support to increase the sheath voltage at the substrate and attract positive ions from the plasma to the substrate. The bias potential effectively determines the kinetic energy of the ions striking the substrate. Thus, the kinetic energy of the positive ions striking the substrate surface is essentially independent of the ion density in an ICP processing system. Accordingly, the surface of the substrate is not damaged by highly energetic ions characteristic of a capacitively-coupled plasma.
Conventional ICP processing systems have a plasma source that may include an antenna or inductive element positioned outside a vacuum chamber and an RF-transmissive window interfacing with the vacuum processing space inside the vacuum chamber. The inductive element is operable for radiating RF energy that is transmitted through the RF-transmissive window to couple with a plasma in the vacuum processing space. The RF-transmissive window is formed of a dielectric material and is incorporated in a vacuum-tight fashion into a structural wall of the vacuum chamber. The surface of the RF-transmissive window is shielded from interactions with the plasma by the deposition shield. The inductive element may be wrapped in a solenoidal fashion about the circumference of a cylindrical RF-transmissive window incorporated into the sidewall of the vacuum chamber. Alternatively, the inductive element may be disposed in a suitable pattern adjacent to a planar RF-transmissive window positioned in the ceiling of the chamber. The thickness of the dielectric material constituting the RF-transmissive window must suffice to withstand the significant forces arising from atmospheric pressure acting over the surface area of the window. In particular, a planar RF-transmissive window must be thick enough to be self-supporting. As a result, a thick RF-transmissive window increases the separation distance between the inductive element and the plasma so as to significantly reduce the amount of RF energy inductively-coupling with the plasma.
In other conventional ICP processing systems, the RF-transmissive window is eliminated or, at the least, minimized by positioning the inductive element within the vacuum chamber. Because the inductive element is closer to the plasma, the efficiency of the inductive coupling of RF energy with the plasma is enhanced over those conventional ICP processing systems that rely upon transmission of RF energy from an external inductive element through a thick RF-transmissive window. However, positioning the inductive element within the vacuum chamber exposes the surfaces of the inductive element to the plasma. Material sputtered from the inductive element may unwantedly deposit on the substrate or other sensitive surfaces within the vacuum chamber. One attempted solution was to form the inductive element from the intended material being deposited onto the substrate as part of the plasma processing. However, this remedy significantly limits the design of the inductive element, restricts the range of materials that can be processed, and may result in plasma-related failures of the inductive element. Further, situating the inductive element within the vacuum chamber significantly increases the complexity of the overall design of the ICP processing system and, if a coolant fluid is circulated through the inductive element, significantly increases the risk of a fluid leak that would compromise the chamber vacuum.
Most planar ICP processing systems incorporate an inductive element that has a footprint commensurate with the area of the substrate to be processed. The semiconductor industry has a need to migrate toward larger area substrates, such as 300 mm silicon wafers and, eventually, larger diameter wafers. Therefore, the inductive element and the RF transmissive window must extend over a large planar area to provide a substantially spatially-uniform distribution of RF energy to a voluminous processing space. As device densities and feature sizes decrease and substrate sizes increase, the ability to produce a highly-dense and uniform plasma throughout the entire processing space increases in importance and becomes increasingly challenging with regard to system design. Because the configuration of the plasma source determines the spatial distribution of the RF energy, the plasma source is the principle factor in determining the plasma density for plasma processing or plasma-assisted processing over the surface area of the substrate and, ultimately, the process throughput and the device yield.
Due to various deficiencies, conventional plasma sources are unable to adequately satisfy the requirements for processing substrates of a large surface area. For example, conventional plasma sources configured to generate a plasma for processing a large-area substrate cannot maximize the energy coupled with the plasma due to, among other factors, the presence of the thick RF-transmissive window. Because the RF-transmissive window forms a self-supporting portion of the chamber wall, its thickness must be able to resist the forces of the external atmospheric pressure. As the surface area of the RF-transmissive window increases that is exposed to the pressure differential between the chamber vacuum and atmospheric pressure, the dielectric material constituting the window must be thickened. As the thickness increases, the RF power must be increased to the inductive element to compensate for the spatial energy losses. Thus, the inability to provide a highly-efficient plasma source for processing large-area substrates is a significant shortcoming that hinders the full development of ICP processing systems.
As a result of the above and other considerations and problems, there remains a need for an inductively-coupled plasma source with efficient and uniform inductive coupling of a density distribution of RF energy for creating a spatially-uniform plasma for the plasma processing of large-area substrates.
The present invention, in one embodiment, provides a system for processing a substrate with a plasma. The processing system comprises a vacuum chamber having a chamber wall which surrounds a vacuum processing space and a gas inlet for introducing a process gas into the vacuum processing space. A substrate support is positioned within the vacuum processing space and is adapted to receive and support the substrate. The chamber wall has an annular opening that receives an annular dielectric trough in a vacuum-tight fashion. Within the vacuum chamber, an inner deposition shield having a plurality of first slots extending therethrough is adjacent an inner dielectric wall of the trough and an outer deposition shield having a plurality of second slots extending therethrough is adjacent an outer dielectric wall of the trough. An inductive element is positioned within the trough and is operably connected to a radiofrequency (RF) energy source. The inductive element couples RF energy through the inner and outer dielectric walls of the trough and the inner and outer deposition shields into the vacuum processing space.
The present invention also provides a plasma source for coupling RF energy from an RF energy source to a plasma confined within a vacuum processing space. The vacuum processing space is surrounded by a chamber wall of a vacuum chamber. The plasma source comprises an annular dielectric trough positioned in an annular opening in the chamber wall in a vacuum-tight fashion. An inner deposition shield is adjacent an inner dielectric wall of the trough and within the vacuum chamber, wherein the inner deposition shield has a plurality of first slots extending therethrough. An outer deposition shield is adjacent the outer dielectric wall and within the vacuum chamber, wherein the outer deposition shield has a plurality of second slots extending therethrough. An inductive element is positioned within the trough and is operably connected to the RF energy source for coupling RF energy through the inner and outer dielectric walls of the trough and the inner and outer deposition shields to the plasma in the vacuum processing space.
The present invention provides an apparatus for generating a dense, uniform plasma in the vacuum processing space by geometrically immersing the plasma source within the plasma. According to the present invention, the immersed plasma sources efficiently couple RF energy to the plasma for enhancing the characteristics of the plasma. The present invention permits the spatial tailoring of the distribution of the RF energy coupled to the plasma to provide a uniform, high density plasma suitable for processing large-area substrates. The present invention provides dense, uniform plasmas while affording a rigid vacuum chamber and locating all water cooling and RF electrical connections outside of the vacuum chamber. Further, the present invention does not require a self-supporting dielectric window that must withstand the significant pressure differential between the interior of the vacuum chamber and atmospheric pressure. These advantages and other advantages of the present invention are set forth in the detailed description hereinbelow.